1. Field of Invention
The present invention relates to an infrared-to-visible converter having combined photoconductive and infrared upconversion phosphors layers, which is used for a solid state image converting device for converting an image of infrared (IR) rays into a visible light image, and more particularly, to an IR-to-visible converter for converting 1.5 .mu.m region IR rays into visible light, such 1.5 .mu.m region IR rays being important in the field of optical fiber communication systems.
2. Description of the Related Art
Recently, the use of 1.5 .mu.m region IR rays, especially in optical fiber communication systems, has increased together with an increasing power of laser diodes in the near-IR region, that is, from 0.7 to 1.5 .mu.m. Infrared upconversion phosphors (IUP) consisting of fluoride or chloride containing Er.sup.3+ is notable as a material for converting 1.5 .mu.m region IR rays into visible light directly. However, a conventional IR converter having IUP only is an up-conversion type device and its conversion efficiency is very low. Since the input intensity of IR rays is usually low, the output visible light of the conventional IR converter is perceivable only in such places as a darkroom.
On the other hand, Japanese Patent Application No. 4-197897 describes a solid-state image converting device for converting a radiation image of IR rays into a visible light image which is perceivable even in a well-lighted room. The device includes a photoconductive (PC) layer and an electroluminescent (EL) layer. However, since the PC layer consists of particles of CdSe compound, the detectable wavelength ranges from 750 nm to 1050 nm, and the problem with this type of solid-state image converting device that is not sensitive to 1.5 .mu.m region IR rays which are widely used in optical fiber communication systems.
Accordingly, a need exists for an IR-to-visible converter which is sensitive to 1.5 .mu.m region IR rays and can convert such IR rays into visible light perceivable even in a well-lighted room.